1. Technical Field
The present invention relates to an antenna device for an RF-ID reader/writer communicating with a plurality of RF-ID tags by using the frequency in the HF band (13.56 MHz band) and a radio communication system using the antenna device.
2. Background Art
In the related art, a circular or rectangular single-winding or multi-winding loop antenna including antenna elements generally arranged in the same plane has been put in practical use as an antenna for an RF-ID reader/writer using the frequency in the HF band (13.56 MHz band).
The loop antenna for an RF-ID reader/writer has largest magnetic field intensity in a direction perpendicular to its opening surface S (direction of center axis). Thus, a loop antenna on the side of an RF-ID tag and a loop antenna for an RF-ID reader/writer are generally arranged so that respective opening surfaces S will be parallel to each other.
For communications with a plurality of RF-ID tags, a plurality of RF-ID tags are arranged in the same plane so that the opening surfaces S will not overlap each other and the opening surfaces S will be parallel to the loop antenna for an RD-ID reader/writer.
In an application where articles each having an extremely small thickness are managed, such as a book management application or a CD/DVD management application, an RF-ID tag cannot be affixed to the spine of a book or a CD/DVD case due to the size of the RF-ID Tag and the physical thickness of the article. As a solution to this problem, a plurality of RF-ID tags have been unavoidably affixed to the front cover or back cover of a target article with narrow intervals (thinness of the article plus α) in an overlapping fashion so that the center axes of tag antennas will be aligned to each other.
In communications with a plurality of RF-ID tags arranged in an overlapping fashion by using a circular or rectangular single-winding or multi-winding loop antenna including antenna elements generally arranged in the same plane, there arises problems including deviations of the antenna resonant frequency due to interference between tags or interference between a tag and the antenna for an reader/writer, or distance attenuation of the magnetic field intensity as a fundamental problem. As a result, since an extremely small number of tags that are read out, communications with all tags are unavailable.
A multi-winding loop antenna described in JP-A-2007-164479 as a solution to this will be described. FIG. 5A is a side view of the main structure of a related art antenna device. FIG. 5B shows distribution of the magnetic field intensity at a distance of a constant height above the outer peripheral side surface in the related art example. In FIG. 5A, a feed element part 11 as a first antenna unit and a parasitic element part 12 as a second antenna unit are electrically grounded at opposed parts thereof to a conductor plate 8 via a grounding metallic pin 13.
The feed element part 11 as the first antenna unit is composed of a multi (n-) winding loop antenna. The initial winding end of a loop is connected to the grounding metallic pin 13. The final winding end of the loop is connected to the resonant/matching circuit. A radiofrequency power is supplied from a reader/writer via a coaxial cable.
The parasitic element part 12 as the second antenna unit is composed of a single-winding loop antenna. The initial winding end of a loop is connected to the grounding metallic pin 13. The final winding end of the loop is connected to the resonant/matching circuit. The loop antenna resonates with a desired frequency and is connected to a matching load (not shown). The magnetic field distribution Hx obtained with this arrangement is shown in FIG. 5B.
The number of elements in the parasitic element part 12 as the second antenna unit is imbalanced compared with the feed element part 11 as the first antenna unit. The magnetic field distribution of the magnetic field Hx in the direction of antenna length (X-axis direction) is imbalanced as shown in FIG. 5B. In this case, it is understood that the magnetic field Hx near the parasitic element part and the first element part of the feed element part ((1) in FIG. 5B) is higher than that of the remaining feed element part.
While a magnetic field intensity above the minimum operation magnetic field Hx (min) of a tag is supplied in communications with a plurality of tags, the magnetic field Hx near the parasitic element part and the first element part of the feed element part ((1) in FIG. 5B) is unnecessarily high, thus reducing the use efficiency of transmit power supplied from the reader/writer.
As described above, in the related art, the number of elements in the parasitic element part as the second antenna unit is imbalanced compared with the feed element part as the first antenna unit. The magnetic field intensity distribution shows an eminent unevenness in part, which reduces the use efficiency of transmit power supplied from the reader/writer.